Bone is the most common site of metastasis for breast cancer (BCa), which causes significant morbidity and mortality in patients with advanced disease. A vicious cycle involving BCa cells and cells in the bone microenvironment results in the activation of osteoclasts and increased bone destruction. Combination chemotherapy and bisphosphonate use for bone lesions provide very little effect on morbidity and survival. Thus, development of newer therapies that can both ameliorate bone destruction and improve survival of patients with metastatic breast disease is needed. A better understanding of the molecular events in BCa bone pathology indicates that receptor activator of nuclear factor kappa-B ligand (RANKL) stimulates the recruitment, differentiation, and activation of osteoclasts by binding to RANK. Osteoprotegerin (OPG) is a decoy receptor that competes with RANK for RANKL, thereby modulating the effects of RANKL. However, during metastases, endogenous OPG levels are markedly reduced. Thus, OPG remains as an effective molecule for future therapies for bone metastasis. The growth of disseminated tumor in the bone further alters the immune milieu through infiltration of myeloid-derived suppressor cells (MDSCs) that dampen the host anti-tumor immune responses. Further, we identified that MDSCs function as osteoclast progenitors directly during this vicious metastatic cascade within the bone both in mouse and in humans, enhancing bone destruction. The current proposal will address three major aspects, namely: osteolytic bone damage, tumor growth and immunosuppression using a genetically-modified stem cell approach targeting osteoclast activation, induction of tumor cell apoptosis using TNF-related apoptosis-inducing ligand (TRAIL), and using gemcitabine for MDSC ablation, respectively, in combination. The central hypothesis of the proposed study is bone-targeted delivery of genetically-engineered OPG, while retaining RANKL binding but abolishing TRAIL binding, in combination with TRAIL therapy together with targeting the MDSC population will significantly decrease osteolytic bone damage, remodel the damaged skeleton, and induce tumor cell apoptosis to improve survival. This hypothesis will be tested using an immunocompetent, preclinical mouse model of bone-disseminated BCa for possible clinical translation. Towards achieving this goal, we have recently established: a) the potential of mesenchymal stem cells (MSC) targeting RANK signaling by using OPG, b) a unique in vivo targeting strategy to enhance bone-specific homing of genetically-engineered MSC, c) the role of MDSCs in forming osteoclasts directly, and d) that depletion of MDSCs enhances anti-tumor Th1 activity in a bone metastatic BCa model. Further, since OPG also binds to TRAIL thereby increasing tumor cells survival, we have: e) identified putative TRAIL binding domain(s) on OPG by homology modeling, and f) developed a mutant OPG (OPGm) that retains RANKL binding but abolishes TRAIL binding and confirmed its biological activity both in vitro and in vivo in bone remodeling. These advances are anticipated to result in a novel treatment option for bone-disseminated BCa.